1. Field of the Invention
This invention relates in general to satellite communication systems and in particular to a novel method and apparatus for synchronizing a ground station in its assigned time slot.
2. Description of the Prior Art
Communication satellite transmission systems are known wherein a time-multiplex arrangement is utilized with each ground station assigned to a particular time slot. For example, German Pat. No. 2,020,094 discloses a system wherein during the initiation of transmission by a new ground station to the TDMA signal first the satellite path is measured utilizing the measuring signal. Such measurement is accomplished such that the incoming station transmits an initial access signal having a period of duration which is usually identical with the duration of the TDMA frame signal. In order to prevent interference between the useful signals and the initial access signal, the initial access signal must be maintained at a level below that of the useful signals. Normally, the access signal is maintained at a level of 25 db below the useful signals. The incoming station receives from the satellite is transmitted access signal and the position of the initial access signal can then be determined by the ground station within the frame and the phase deviation from the desired phase of the subsequent signals which are to be sent can be obtained from this phase measurement. Generally, only the preamble of the ground station is first transmitted utilizing the phase found in this manner, and the initiation of this signal is adjusted with a phase control device in the ground station over the control loop of which the satellite forms a part until the preamble is accurately adjusted in phase to the desired phase. Thereafter, periodically during the time slot for the particular station the entire transmission of the station follows the preamble. The phase control device for the transmission is relatively inexpensive in that it is also required to maintain the transmissions in their proper time slots under steady operation. This is necessary even though a satellite which moves synchronously with the earth rotation, does not remain absolutely stationary but has noticeable movements in all three spacial planes.
In the systems of satellite communication transmission systems, utilizing the TDMA technique, it is desirable to permit the transmission of bursts as quickly and accurately as possible after the initial phase-deviation measurement of the initial access signal has been accomplished. The time, which is required for carrying out the initial access depends on the transmission time of the initial access signal in one case and in the other case on the required time duration to accomplish phase correction after which the complete transmission including intelligence can occur.
In presently operating systems, the transmission time of the initial access signal generally requires three seconds and this time cannot as a practical matter, be shortened due to the relatively long transit time which is in the order of magnitude of a few hundred milliseconds which is required for the initial access signal to pass to the satellite and be received back at the transmitting ground station. The time required for the phase correction before the full intelligent burst can be transmitted, however depends on the accuracy of the phase-deviation measurement at the receiving side and the time required for the accessing station to correct the desired phase. The precision depends on the structure of the initial access signal, on the time available for the initial access process and on the signal level difference between the useful signals and the initial access signal. In the Intelsat IV test system, the minimum phase measuring accuracy .+-. 4 microseconds has been determined.
As mentioned above, after the initial transmission phase correction has been made, only the burst preamble is transmitted and the transmission phase regulation is accomplished with this signal. It has been determined with a test system that there are maximum phase-change speeds that can be utilized for safety reasons and it has been determined that the phase correction speed must be limited and accomplished within the transmission phase fine adjustment. It has been determined that the phase correction steps should not exceed one unit per frame. It has also been determined that maximum of seven correction steps per three hundred ms dead time are to be accomplished when considering the transit time of the satellite loop. If the minimum measuring accuracy of measurement of the phase-deviation of the initial access signal is .+-. 4 microseconds, a time up to 13 seconds will be required for the burst transmission phase adjustment. This value may be greater if possible phase changes must be compensated which occurred during this time due to generator drift.